I'm using KDEnlive to edit multiple videotracks. I have the tracks synched, but they are different lengths. In a preview of the video I can sort-of live edit by switching different video-channels on and off. The topmost active video will show.

What I would like to do, is do this for my final edit. I don't want to cut the video's into pieces, because I might change my mind about where I want to transition over to another channel. So I'd like to mark a spot on a channel where it changes to another channel (perhaps with a transition) and play around with these until I have the edit I want.

I imagine it's possible to transition from one channel to the other without actually deleting parts of the video and cutting channels up into clips?

I would keep the master tracks for the reference and copy/paste edit pieces into the tracks above. This way if you want to re-edit you just will have to change the topmost track and the reference always stays in place.

Here's how I would do it:

On the top video, use CairoBlend in combination with keyframes. First, add CairoBlend on the top-most track and stretch it to fit the length of your video.

Now, click on CairoBlend itself. You can add keyframes (use the + to add a keyframe, then advance with the up/down arrows in the upper right of the effect). Setting the opacity to zero will make the top video transparent and vice-versa. To do this the way (I think) you want is going to require two keyframes next to each other for each transition to the other video; otherwise you will end up with a fade effect instead of an instant switch to the other video.

Do it like this (K = keyframe, 0 and 100 = opacity):

K0......top video......K0 K100......top video......K100 K0......top video......K0 K100......top video......K100

As long as you ALWAYS keep the top video BETWEEN two keyframes with either 0 or 100, the video segments will instantly switch back and forth. Each time you need to make a video switch, you will have a zero opacity keyframe next to a 100 opacity keyframe. I hope that makes sense.

Edited to add: If you are doing this with more than two videos, you can add another CairoBlend transition to the next video down and repeat the keyframe process though it might get a bit confusing. You can easily move keyframes just by dragging them along the line in the effect box when you have the transition highlighted on the timeline.